Sometimes, instead of adding a drop of water to a cask strength or high proof whisky (something I often
DO NOT do), I simply add another whisky!
Example: Try topping up the Glenlivet NÃ durra with a touch of Caol Ila Cask Strength... You'll be amazed at how the Islay characteristics of the latter accentuate, by highlighting the contrast between the two malts, the sweetness of the former.
One cask strength single malt I do regularly add a minute quantity of water to, however, is The Macallan as, in this case, I find the water tempers the minimal harshness of the spirit perfectly.